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The Fermi catalogue contains about 2000 unassociated $\gamma$-ray sources. Some of them were recently identified as pulsars, including so called redbacks and black widows, which are millisecond pulsars in tight binary systems with non- and partially-degenerate low-mass stellar companions irradiated by the pulsar wind. We study a likely optical and X-ray counterpart of the Fermi source 4FGL J2054.2+6904 proposed earlier as a pulsar candidate. We use archival optical data as well as Swift/XRT and SRG/eROSITA X-ray data to clarify its nature. Using Zwicky Transient Facility data in $g$ and $r$ bands spanning over 4.7 years, we find a period of $\approx$7.5 h. The folded light curve has a smooth sinusoidal shape with the peak-to-peak amplitude of $\approx$0.4 mag. The spectral fit to the optical spectral energy distribution of the counterpart candidate gives the star radius of 0.5$\pm$0.1$R_\odot$ and temperature of 5500$\pm$300 K implying a G2--G9-type star. Its X-ray spectrum is well fitted by an absorbed power law with the photon index of 1.0$\pm$0.3 and unabsorbed flux of $\approx 2\times10^{-13}$ erg s$^{-1}$ cm$^{-2}$. All the properties of 4FGL J2054.2$+$6904 and its presumed counterpart suggest that it is a member of the redback family., Comment: Accepted for publication in MNRAS, 6 pages, 6 figures

We present the results of optical spectroscopy of stellar companions to three binary millisecond pulsars, PSRs J0621$+$2514, J2317$+$1439 and J2302$+$4442, obtained with the Gran Telescopio Canarias. The spectrum of the J0621$+$2514 companion shows a blue continuum and prominent Balmer absorption lines. The latter are also resolved in the spectrum of the J2317$+$1439 companion, showing that both are DA-type white dwarfs. No spectral features are detected for the J2302$+$4442 companion, however, its broadband magnitudes and the spectral shape of the continuum emission imply that this is also a DA-type white dwarf. Based on the spectral analyses, we conclude that the companions of J0621$+$2514 and J2317$+$1439 are relatively hot, with effective temperatures $T_{\rm eff}$$=$8600$\pm$200 and 9600$\pm$2000~K, respectively, while the J2302$+$4442 companion is significantly cooler, $T_{\rm eff}$$<$6000~K. We also estimated the distance to J0621$+$2514 of 1.1$\pm$0.3 kpc and argue that its companion and the companion of J2317$+$1439 are He-core white dwarfs providing constraints on their cooling ages of $\lesssim$2 Gyr., Comment: 9 pages, 8 figures. Paper accepted to MNRAS

An unidentified $\gamma$-ray source 4FGL J1838.2+3223 has been proposed as a pulsar candidate. We present optical time-series multi-band photometry of its likely optical companion obtained with the 2.1-m telescope of Observatorio Astron\'omico Nacional San Pedro M\'artir, Mexico. The observations and the data from the Zwicky Transient Facility revealed the source brightness variability with a period of $\approx$4.02 h likely associated with the orbital motion of the binary system. The folded light curves have a single sine-like peak per period with an amplitude of about three magnitude accompanied by fast sporadic flares up to one magnitude level. We reproduce them modelling the companion heating by the pulsar. As a result, the companion side facing the pulsar is strongly heated up to 11300$\pm$400 K, while the temperature of its back side is only 2300$\pm$700 K. It has a mass of 0.10$\pm$0.05 ${\rm M}_\odot$ and underfills its Roche lobe with a filling factor of $0.60^{+0.10}_{-0.06}$. This implies that 4FGL J1838.2+3223 likely belongs to the `spider' pulsar family. The estimated distance of $\approx$3.1 kpc is compatible with Gaia results. We detect a flare from the source in X-rays and ultraviolet using Swift archival data and another one in X-rays with the eROSITA all-sky survey. Both flares have X-ray luminosity of $\sim$10$^{34}$ erg s$^{-1}$ which is two orders of magnitude higher than the upper limit in quiescence obtained from eROSITA assuming spectral shape typical for spider pulsars. If the spider interpretation is correct, these flares are among the strongest flares observed from non-accreting spider pulsars., Comment: 10 pages, 11 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

PSR J1641+8049 is a 2 ms black widow pulsar with the 2.2 h orbital period detected in the radio and $\gamma$-rays. We performed new phase-resolved multi-band photometry of PSR J1641+8049 using the OSIRIS instrument at the Gran Telescopio Canarias. The obtained data were analysed together with the new radio-timing observations from the Canadian Hydrogen Intensity Mapping Experiment (CHIME), the X-ray data from the Spectrum-RG/eROSITA all-sky survey, and all available optical photometric observations. An updated timing solution based on CHIME data is presented, which accounts for secular and periodic modulations in pulse dispersion. The system parameters obtained through the light curve analysis, including the distance to the source 4.6-4.8 kpc and the orbital inclination 56-59 deg, are found to be consistent with previous studies. However, the optical flux of the source at the maximum brightness phase faded by a factor of $\sim$2 as compared to previous observations. Nevertheless, the face of the J1641+8049 companion remains one of the most heated (8000-9500 K) by a pulsar among the known black widow pulsars. We also report a new estimation on the pulsar proper motion of $\approx$2 mas yr$^{-1}$, which yields a spin down luminosity of $\approx$4.87$\times 10^{34}$ ergs s$^{-1}$ and a corresponding heating efficiency of the companion by the pulsar of 0.3-0.7. The pulsar was not detected in X-rays implying its X-ray-luminosity was <3 $\times$ 10$^{31}$ erg s$^{-1}$ at the date of observations., Comment: Accepted for publication in MNRAS

XMM-Newton observations of the middle-aged radio-quiet $\gamma$-ray pulsar J0554+3107 allowed us, for the first time, firmly identify it in X-rays by detection of pulsations with the pulsar period. In the 0.2-2 keV band, the pulse profile shows two peaks separated by about a half of the rotation phase with the pulsed fraction of $25 \pm 6$ per cent. The profile and spectrum in this band can be mainly described by thermal emission from the neutron star with the hydrogen atmosphere, dipole magnetic field of $\sim 10^{13}$ G and non-uniform surface temperature. Non-thermal emission from the pulsar magnetosphere is marginally detected at higher photon energies. The spectral fit with the atmosphere+power law model implies that J0554+3107 is a rather heavy and cool neutron star with the mass of 1.6-2.1 $M_\odot$, the radius of $\approx 13$ km and the redshifted effective temperature of $\approx 50$ eV. The spectrum shows an absorption line of unknown nature at $\approx 350$ eV. Given the extinction-distance relation, the pulsar is located at $\approx 2$ kpc and has the redshifted bolometric thermal luminosity of $\approx 2 \times 10^{32}$ erg s$^{-1}$. We discuss cooling scenarios for J0554+3107 considering plausible equations of state of super-dense matter inside the star, different compositions of the heat-blanketing envelope and various ages., Comment: 13 pages, 13 figures. Accepted for publication in MNRAS

The 840 kyr old pulsar PSR J1957+5033, detected so far only in $\gamma$- and X-rays, is a nearby and rather cool neutron star with a temperature of 0.2--0.3 MK, a distance of $\la$1 kpc, and a small colour reddening excess $E(B-V) \approx 0.03$. These properties make it an ideal candidate to detect in the optical to get additional constraints on its parameters. We thus performed the first deep optical observations of the pulsar with the 10.4-meter Gran Telescopio Canarias in the $g'$ band and found its possible counterpart with $g'=27.63\pm 0.26$. The counterpart candidate position is consistent with the X-ray coordinates of the pulsar within the 0.5 arcsec accuracy. Assuming that this is the real counterpart, we analysed the pulsar X-ray spectrum together with the derived optical flux density. As a result, we found that the thermal emission from the bulk surface of the cooling neutron star can significantly contribute to its optical flux. Our multi-wavelength spectral analysis favours the pulsar nature of the detected optical source, since it provides physically adequate parameters of the pulsar emission. We show that the optical data can provide new constraints on the pulsar temperature and distance., Comment: 7 pages, 3 figures, published in MNRAS

We analyze new XMM-Newton and archival Chandra observations of the middle-aged $\gamma$-ray radio-quiet pulsar J1957+5033. We detect, for the first time, X-ray pulsations with the pulsar spin period of the point-like source coinciding by position with the pulsar. This confirms the pulsar nature of the source. In the 0.15--0.5 keV band, there is a single pulse per period and the pulsed fraction is $\approx18\pm6$ per cent. In this band, the pulsar spectrum is dominated by a thermal emission component that likely comes from the entire surface of the neutron star, while at higher energies ($\gtrsim0.7$ keV) it is described by a power law with the photon index $\Gamma \approx 1.6$. We construct new hydrogen atmosphere models for neutron stars with dipole magnetic fields and non-uniform surface temperature distributions with relatively low effective temperatures. We use them in the spectral analysis and derive the pulsar average effective temperature of $\approx(2-3)\times10^5$ K. This makes J1957+5033 the coldest among all known thermally emitting neutron stars with ages below 1 Myr. Using the interstellar extinction--distance relation, we constrain the distance to the pulsar in the range of 0.1--1 kpc. We compare the obtained X-ray thermal luminosity with those for other neutron stars and various neutron star cooling models and set some constraints on latter. We observe a faint trail-like feature, elongated $\sim 8$ arcmin from J1957+5033. Its spectrum can be described by a power law with a photon index $\Gamma=1.9\pm0.5$ suggesting that it is likely a pulsar wind nebula powered by J1957+5033., Comment: 14 pages, 13 figures, published in MNRAS

We report detection of PSR B0656$+$14 with the Gran Telescopio Canarias in narrow optical $F657$, $F754$, $F802$, and $F902$ and near-infrared $JHK_s$ bands. The pulsar detection in the $K_s$ band extends its spectrum to 2.2 $\mu$m and confirms its flux increase towards the infrared. We also present a thorough analysis of the optical spectrum obtained by us with the VLT. For a consistency check, we revised the pulsar near-infrared and narrow-band photometry obtained with the \textit{HST}. We find no narrow spectral lines in the optical spectrum. We compile available near-infrared-optical-UV and archival 0.3-20keV X-ray data and perform a self-consistent analysis of the rotation phase-integrated spectrum of the pulsar using unified spectral models. The spectrum is best fitted by the four-component model including two blackbodies, describing the thermal emission from the neutron star surface and its hot polar cap, the broken power-law, originating from the pulsar magnetosphere, and an absorption line near $\sim$0.5 keV detected previously. The fit provides better constraints on the model parameters than using only a single spectral domain. The derived surface temperature is $T_{NS}^{\infty}=7.9(3)\times10^5$K. The intrinsic radius (7.8-9.9 km) of the emitting region is smaller than a typical neutron star radius (13km) and suggests a nonuniform temperature distribution over the star surface. In contrast, the derived radius of the hot polar cap is about twice as large as the `canonical' one. The spectrum of the nonthermal emission steepens from the optical to X-rays and has a break near 0.1 keV. The X-ray data suggest the presence of another absorption line near 0.3keV., Comment: 18 pages, 9 figures, Accepted for publication in MNRAS

We present observations of the pulsar-wind nebula (PWN) region of SNR 0540-69.3. The observations were made with the Atacama Compact Array (ACA) in Bands 4 and 6. We also add radio observations from the Australia Compact Array at 3 cm. For 1.449-233.50 GHz, we obtain a synchrotron spectrum F-nu proportional to nu(-alpha nu) with the spectral index alpha(nu) = 0.17 +/- 0.02. To conclude how this joins the synchrotron spectrum at higher frequencies, we include hitherto unpublished AKARI mid-infrared data, and evaluate published data in the ultraviolet (UV), optical, and infrared (IR). In particular, some broad-band filter data in the optical must be discarded from our analysis due to contamination by spectral line emission. For the UV/IR part of the synchrotron spectrum, we arrive at alpha(nu) = 0.87(-0.10)(+0.08). There is room for 2.5 x 10(-3) M-circle dot of dust with a temperature of similar to 55 K if there are dual breaks in the synchrotron spectrum, one around similar to 9 x 10(10) Hz and another at similar to 2 x 10(13) Hz. The spectral index then changes at similar to 9 x 10(10) Hz from alpha(nu) = 0.14 +/- 0.07 in the radio to alpha(nu) = 0.35(+0.05)(-0.07) in the millimetre-to-far-IR range. The ACA Band 6 data marginally resolve the PWN. In particular, the strong emission similar to 1 ''.5 south-west of the pulsar, seen at other wavelengths, and resolved in the 3 cm data with its 0 ''.8 spatial resolution, is also strong in the millimetre range. The ACA data clearly reveal the supernova remnant shell similar to 20-35 arcsec west of the pulsar, and for the shell we derive alpha(nu )= 0.64 +/- 0.05 for the range 8.6-145 GHz.

Ages and thermal luminosities of neutron stars, inferred from observations, can be interpreted with the aid of the neutron star cooling theory to gain information on the properties of superdense matter in neutron-star interiors. We present a survey of estimated ages, surface temperatures and thermal luminosities of middle-aged neutron stars with relatively weak or moderately strong magnetic fields, which can be useful for these purposes. The catalogue includes results selected from the literature, supplemented with new results of spectral analysis of a few cooling neutron stars. The data are compared with the theory. We show that overall agreement of theoretical cooling curves with observations improves substantially for models where neutron superfluidity in stellar core is weak., Comment: 23 pages, 3 figures, 2 tables, accepted by MNRAS. In v.2, acknowledgements are supplemented

We present observations of the pulsar-wind nebula (PWN) region ofSNR 0540-69.3. The observations were made with the Atacama Compact Array (ACA) in Bands 4 and 6. We also add radio observations from the Australia Compact Array (ATCA) at 3 cm. For 1.449 - 233.50 GHz we obtain a synchrotron spectrum $F_{\nu} \propto \nu^{-\alpha_{\nu}}$, with the spectral index $\alpha_{\nu} = 0.17\pm{0.02}$. To conclude how this joins the synchrotron spectrum at higher frequencies we include hitherto unpublished AKARI mid-infrared data, and evaluate published data in the ultraviolet (UV), optical and infrared (IR). In particular, some broad-band filter data in the optical must be discarded from our analysis due to contamination by spectral line emission. For the UV/IR part of the synchrotron spectrum, we arrive at $\alpha_{\nu} = 0.87^{+0.08}_{-0.10}$. There is room for $2.5\times10^{-3}$ solar masses of dust with temperature $\sim 55$ K if there are dual breaks in the synchrotron spectrum, one around $\sim 9\times10^{10}$ Hz, and another at $\sim 2\times10^{13}$ Hz. The spectral index then changes at $\sim 9\times10^{10}$ Hz from $\alpha_{\nu} = 0.14\pm0.07$ in the radio, to $\alpha_{\nu} = 0.35^{-0.07}_{+0.05}$ in the millimetre to far-IR range. The ACA Band 6 data marginally resolves the PWN. In particular, the strong emission 1.5" south-west of the pulsar, seen at other wavelengths, and resolved in the 3-cm data with its 0.8" spatial resolution, is also strong in the millimeter range. The ACA data clearly reveal the supernova remnant shell 20-35 arcsec west of the pulsar, and for the shell we derive $\alpha_{\nu} = 0.64\pm{0.05}$ for the range $8.6-145$~GHz., Comment: MNRAS, accepted

We report on the results of Chandra X-ray observations of the $\gamma$-ray radio-quiet pulsar J1932+1916. We confirm the previous detection of the pulsar counterpart and its pulsar wind nebula in X-rays from low spatial resolution data obtained by Suzaku and Swift. The Chandra data with much better spatial resolution resolved the fine structure of the nebula in the pulsar vicinity, which can be interpreted as jets bent by the ram pressure. The size of this compact part is about 30'', and it is surrounded by a weak asymmetric diffusive emission extended up to $\approx$ 2.8'. The X-ray spectra of the pulsar, the compact and extended parts of the nebula can be described by the power-law models with photon indexes of 0.44, 2.34 and 2.14, respectively. The actual pulsar's X-ray flux is several times weaker than it was obtained with Suzaku and Swift where it was dominated by the unresolved nebula. We discuss possible associations of J1932+1916 with the nearby supernova remnant G54.4-0.3.

We report on multi-band photometric observations of four binary millisecond pulsars with the Gran Telescopio Canarias. The observations led to detection of binary companions to PSRs J1630+3734, J1741+1351 and J2042+0246 in the Sloan g', r' and i' bands. Their magnitudes in the r' band are $\approx$24.4, 24.4 and 24.0, respectively. We also set a 3$\sigma$ upper limit on the brightness of the PSR J0557+1550 companion in the r' band of $\approx$25.6 mag. Combining the optical data with the radio timing measurements and white dwarf cooling models, we show that the detected companions are cool low-mass white dwarfs with temperatures and ages in the respective ranges of (4-7)$\times 10^3$ K and 2-5 Gyr. All the detected white dwarfs are found to likely have either pure hydrogen or mixed helium-hydrogen atmospheres., Comment: Accepted for publication in MNRAS

We analyse archival optical data on the binary companion to the millisecond pulsar \j1045\ obtained with the Very Large Telescope. A possible optical counterpart candidate %source position coincides with the pulsar coordinates is found at the pulsar position. Its magnitudes are $V>26.4$, $R=25.7(2)$ and $I=25.4(2)$. The data are compared with white dwarf evolutionary models. Depending on still poorly known distance to the pulsar, two alternative interpretations are possible. For the radio timing parallax distance, the candidate can be a very cool and old white dwarf with the hydrogen atmosphere and the temperature of $< 3000$ K and the age of $> 5$ Gyr. The dispersion measure distance suggests a younger ($\sim$ 2 Gyr) and hotter ($\sim$ 6000 K) white dwarf, whose atmosphere composition remains unknown.

Ages and thermal luminosities of neutron stars, inferred from observations, can be interpreted with the aid of the neutron star cooling theory to gain information on the properties of superdense matter in neutron-star interiors. We present a survey of estimated ages, surface temperatures and thermal luminosities of middle-aged neutron stars with relatively weak or moderately strong magnetic fields, which can be useful for these purposes. The catalogue includes results selected from the literature, supplemented with new results of spectral analysis of a few cooling neutron stars. The data are compared with the theory. We show that overall agreement of theoretical cooling curves with observations improves substantially for models where neutron superfluidity in stellar core is weak., Comment: 23 pages, 3 figures, 2 tables, accepted by MNRAS. In v.2, acknowledgements are supplemented

We present observations of the pulsar-wind nebula (PWN) region ofSNR 0540-69.3. The observations were made with the Atacama Compact Array (ACA) in Bands 4 and 6. We also add radio observations from the Australia Compact Array (ATCA) at 3 cm. For 1.449 - 233.50 GHz we obtain a synchrotron spectrum $F_{\nu} \propto \nu^{-\alpha_{\nu}}$, with the spectral index $\alpha_{\nu} = 0.17\pm{0.02}$. To conclude how this joins the synchrotron spectrum at higher frequencies we include hitherto unpublished AKARI mid-infrared data, and evaluate published data in the ultraviolet (UV), optical and infrared (IR). In particular, some broad-band filter data in the optical must be discarded from our analysis due to contamination by spectral line emission. For the UV/IR part of the synchrotron spectrum, we arrive at $\alpha_{\nu} = 0.87^{+0.08}_{-0.10}$. There is room for $2.5\times10^{-3}$ solar masses of dust with temperature $\sim 55$ K if there are dual breaks in the synchrotron spectrum, one around $\sim 9\times10^{10}$ Hz, and another at $\sim 2\times10^{13}$ Hz. The spectral index then changes at $\sim 9\times10^{10}$ Hz from $\alpha_{\nu} = 0.14\pm0.07$ in the radio, to $\alpha_{\nu} = 0.35^{-0.07}_{+0.05}$ in the millimetre to far-IR range. The ACA Band 6 data marginally resolves the PWN. In particular, the strong emission 1.5" south-west of the pulsar, seen at other wavelengths, and resolved in the 3-cm data with its 0.8" spatial resolution, is also strong in the millimeter range. The ACA data clearly reveal the supernova remnant shell 20-35 arcsec west of the pulsar, and for the shell we derive $\alpha_{\nu} = 0.64\pm{0.05}$ for the range $8.6-145$~GHz., Comment: MNRAS, accepted

We analyse archival optical data on the binary companion to the millisecond pulsar \j1045\ obtained with the Very Large Telescope. A possible optical counterpart candidate %source position coincides with the pulsar coordinates is found at the pulsar position. Its magnitudes are $V>26.4$, $R=25.7(2)$ and $I=25.4(2)$. The data are compared with white dwarf evolutionary models. Depending on still poorly known distance to the pulsar, two alternative interpretations are possible. For the radio timing parallax distance, the candidate can be a very cool and old white dwarf with the hydrogen atmosphere and the temperature of $< 3000$ K and the age of $> 5$ Gyr. The dispersion measure distance suggests a younger ($\sim$ 2 Gyr) and hotter ($\sim$ 6000 K) white dwarf, whose atmosphere composition remains unknown.

We report on the results of Chandra X-ray observations of the $\gamma$-ray radio-quiet pulsar J1932+1916. We confirm the previous detection of the pulsar counterpart and its pulsar wind nebula in X-rays from low spatial resolution data obtained by Suzaku and Swift. The Chandra data with much better spatial resolution resolved the fine structure of the nebula in the pulsar vicinity, which can be interpreted as jets bent by the ram pressure. The size of this compact part is about 30'', and it is surrounded by a weak asymmetric diffusive emission extended up to $\approx$ 2.8'. The X-ray spectra of the pulsar, the compact and extended parts of the nebula can be described by the power-law models with photon indexes of 0.44, 2.34 and 2.14, respectively. The actual pulsar's X-ray flux is several times weaker than it was obtained with Suzaku and Swift where it was dominated by the unresolved nebula. We discuss possible associations of J1932+1916 with the nearby supernova remnant G54.4-0.3.

We report on multi-band photometric observations of four binary millisecond pulsars with the Gran Telescopio Canarias. The observations led to detection of binary companions to PSRs J1630+3734, J1741+1351 and J2042+0246 in the Sloan g', r' and i' bands. Their magnitudes in the r' band are $\approx$24.4, 24.4 and 24.0, respectively. We also set a 3$\sigma$ upper limit on the brightness of the PSR J0557+1550 companion in the r' band of $\approx$25.6 mag. Combining the optical data with the radio timing measurements and white dwarf cooling models, we show that the detected companions are cool low-mass white dwarfs with temperatures and ages in the respective ranges of (4-7)$\times 10^3$ K and 2-5 Gyr. All the detected white dwarfs are found to likely have either pure hydrogen or mixed helium-hydrogen atmospheres., Comment: Accepted for publication in MNRAS

We present optical time-resolved multi-band photometry of the black widow binary millisecond pulsar J2052+1219 using direct-imaging observations with the 2.1m telescope of Observatorio Astronomico Nacional San Pedro Martir, Mexico (OAN-SPM). The observations revealed a variable optical source whose position and periodicity P = 2.752h coincide with the pulsar coordinates and the orbital period obtained from radio timing. This allowed us to identify it with the binary companion of the pulsar. We reproduce light curves of the source modelling the companion heating by the pulsar and accounting for the system parameters obtained from the radio data. As a result, we independently estimate the distance to the system of 3.94(16) kpc, which agrees with the dispersion measure distance. The companion star size is 0.12-0.15 Rsun, close to filling its Roche lobe. It has a surface temperature difference of about 3000 K between the side facing the pulsar and the back side. We summarise characteristics of all black widow systems studied in the optical and compare them with the PSR J2052+1219 parameters derived from our observations., Comment: Accepted for publication in MNRAS; 10 pages, 5 figures

We have analyzed archival XMM-Newton and Chandra observations of the gamma-ray radio-quiet pulsar J1826-1256 and its pulsar wind nebula. The pulsar spectrum can be described by a power-law model with a photon index $\Gamma\approx1$. We find that the nebular spectrum softens with increasing distance from the pulsar, implying synchrotron cooling. The empirical interstellar absorption-distance relation gives a distance of $\approx3.5$ kpc to J1826-1256. We also discuss the nebula geometry and association between the pulsar, the very high energy source HESS J1826-130, the supernova remnant candidate G18.45-0.42 and the open star cluster Bica 3., Comment: Accepted for publication in MNRAS

We report detection of the binary companion to the millisecond pulsar J1741+1351 with the Gran Telescopio Canarias. The optical source position coincides with the pulsar coordinates and its magnitudes are g' = 24.84(5), r' = 24.38(4) and i' = 24.17(4). Comparison of the data with the white dwarf evolutionary models shows that the source can be a He-core white dwarf with a temperature of $\approx 6000$ K and a mass of $\approx 0.2$ M$_{\odot}$. The latter is in excellent agreement with the companion mass obtained from the radio timing solution for PSR J1741+1351., Comment: Submitted to Journal of Physics: Conference Series

We report the detection of the likely companion of the binary millisecond pulsar J0740+6620 with the Gran Telescopio Canarias in the $r'$ and $i'$ bands. The position of the detected starlike source coincides with the pulsar coordinates within the 1$\sigma$ uncertainty of $\approx$ 0.2 arcsec. Its magnitudes are $r'=26.51\pm0.17$ and $i'=25.49\pm0.15$. Comparing the data with the white dwarf cooling tracks suggests that it can be an ultracool helium-atmosphere white dwarf with the temperature $\lesssim$ 3500 K and cooling age $\gtrsim$ 5 Gyr. The age is consistent with the pulsar characteristic age corrected for kinematic effects. This is the reddest source among known white dwarf companions of millisecond pulsars. Detection of the source in other bands would be useful to clarify its properties and nature., Comment: Accepted for publication in MNRAS

We have analyzed archival XMM-Newton and Chandra observations of the gamma-ray radio-quiet pulsar J1826-1256 and its pulsar wind nebula. The pulsar spectrum can be described by a power-law model with a photon index $\Gamma\approx1$. We find that the nebular spectrum softens with increasing distance from the pulsar, implying synchrotron cooling. The empirical interstellar absorption-distance relation gives a distance of $\approx3.5$ kpc to J1826-1256. We also discuss the nebula geometry and association between the pulsar, the very high energy source HESS J1826-130, the supernova remnant candidate G18.45-0.42 and the open star cluster Bica 3., Comment: Accepted for publication in MNRAS

We report detection of the binary companion to the millisecond pulsar J1741+1351 with the Gran Telescopio Canarias. The optical source position coincides with the pulsar coordinates and its magnitudes are g' = 24.84(5), r' = 24.38(4) and i' = 24.17(4). Comparison of the data with the white dwarf evolutionary models shows that the source can be a He-core white dwarf with a temperature of $\approx 6000$ K and a mass of $\approx 0.2$ M$_{\odot}$. The latter is in excellent agreement with the companion mass obtained from the radio timing solution for PSR J1741+1351., Comment: Submitted to Journal of Physics: Conference Series

We report the detection of the likely companion of the binary millisecond pulsar J0740+6620 with the Gran Telescopio Canarias in the $r'$ and $i'$ bands. The position of the detected starlike source coincides with the pulsar coordinates within the 1$\sigma$ uncertainty of $\approx$ 0.2 arcsec. Its magnitudes are $r'=26.51\pm0.17$ and $i'=25.49\pm0.15$. Comparing the data with the white dwarf cooling tracks suggests that it can be an ultracool helium-atmosphere white dwarf with the temperature $\lesssim$ 3500 K and cooling age $\gtrsim$ 5 Gyr. The age is consistent with the pulsar characteristic age corrected for kinematic effects. This is the reddest source among known white dwarf companions of millisecond pulsars. Detection of the source in other bands would be useful to clarify its properties and nature., Comment: Accepted for publication in MNRAS

We present optical time-resolved multi-band photometry of the black widow binary millisecond pulsar J2052+1219 using direct-imaging observations with the 2.1m telescope of Observatorio Astronomico Nacional San Pedro Martir, Mexico (OAN-SPM). The observations revealed a variable optical source whose position and periodicity P = 2.752h coincide with the pulsar coordinates and the orbital period obtained from radio timing. This allowed us to identify it with the binary companion of the pulsar. We reproduce light curves of the source modelling the companion heating by the pulsar and accounting for the system parameters obtained from the radio data. As a result, we independently estimate the distance to the system of 3.94(16) kpc, which agrees with the dispersion measure distance. The companion star size is 0.12-0.15 Rsun, close to filling its Roche lobe. It has a surface temperature difference of about 3000 K between the side facing the pulsar and the back side. We summarise characteristics of all black widow systems studied in the optical and compare them with the PSR J2052+1219 parameters derived from our observations., Comment: Accepted for publication in MNRAS; 10 pages, 5 figures

In this paper, we demonstrate the connection between a magnetic storage ring with additional sextupole fields set so that the x and y chromaticities vanish and the maximizing of the lifetime of in-plane polarization (IPP) for a 0.97-GeV/c deuteron beam. The IPP magnitude was measured by continuously monitoring the down-up scattering asymmetry (sensitive to sideways polarization) in an in-beam, carbontarget polarimeter and unfolding the precession of the IPP due to the magnetic anomaly of the deuteron. The optimum operating conditions for a long IPP lifetime were made by scanning the field of the storage ring sextupole magnet families while observing the rate of IPP loss during storage of the beam. The beam was bunched and electron cooled. The IPP losses appear to arise from the change of the orbit circumference, and consequently the particle speed and spin tune, due to the transverse betatron oscillations of individual particles in the beam. The effects of these changes are canceled by an appropriate sextupole field setting., QC 20180222

We report detection of the binary companion to the millisecond pulsar J2302+4442 based on the deep observations performed with the Gran Telescopio Canarias. The observations revealed an optical source with $r'$ = 23.33$\pm$0.02 and $i'$=23.08$\pm$0.02, whose position coincides with the pulsar radio position. By comparing the source colour and magnitudes with the white dwarf cooling predictions, we found that it likely represents a He or CO-core white dwarf and estimated its mass of 0.52$^{+0.25}_{-0.19}$ M$_{\odot}$ and effective temperature of 6300$^{+1000}_{-800}$ K. Combining our results with the radio timing measurements, we set constraints on the binary system inclination angle $i=73^{+6}_{-5}$ degrees and the pulsar mass $m_p=3.1^{+2.7}_{-2.0}$ M$_{\odot}$., Comment: Accepted for publication in MNRAS

Using the SDSS and Pan-STARRS1 survey data, we found a likely companion of the recently discovered binary $\gamma$-ray radio-loud millisecond pulsar J0621+2514. Its visual brightness is about 22 mag. The broad band magnitudes and colours suggest that this is a white dwarf. Comparing the data with various white dwarfs evolutionary tracks, we found that it likely belongs to a class of He-core white dwarfs with a temperature of about 10 000 K and a mass of $\lesssim 0.5$M$_\odot$. For a thin hydrogen envelope of the white dwarf its cooling age is $\lesssim0.5$ Gyr which is smaller than the pulsar characteristic age of 1.8 Gyr. This may indicate that the pulsar age is overestimated. Otherwise, this may be explained by the presence of a thick hydrogen envelope or a low metallicity of the white dwarf progenitor., Comment: 5 pages, 4 figures, accepted for publication in PASA

We report detection of the binary companion to the millisecond pulsar J2302+4442 based on the deep observations performed with the Gran Telescopio Canarias. The observations revealed an optical source with $r'$ = 23.33$\pm$0.02 and $i'$=23.08$\pm$0.02, whose position coincides with the pulsar radio position. By comparing the source colour and magnitudes with the white dwarf cooling predictions, we found that it likely represents a He or CO-core white dwarf and estimated its mass of 0.52$^{+0.25}_{-0.19}$ M$_{\odot}$ and effective temperature of 6300$^{+1000}_{-800}$ K. Combining our results with the radio timing measurements, we set constraints on the binary system inclination angle $i=73^{+6}_{-5}$ degrees and the pulsar mass $m_p=3.1^{+2.7}_{-2.0}$ M$_{\odot}$., Comment: Accepted for publication in MNRAS

Using the SDSS and Pan-STARRS1 survey data, we found a likely companion of the recently discovered binary $\gamma$-ray radio-loud millisecond pulsar J0621+2514. Its visual brightness is about 22 mag. The broad band magnitudes and colours suggest that this is a white dwarf. Comparing the data with various white dwarfs evolutionary tracks, we found that it likely belongs to a class of He-core white dwarfs with a temperature of about 10 000 K and a mass of $\lesssim 0.5$M$_\odot$. For a thin hydrogen envelope of the white dwarf its cooling age is $\lesssim0.5$ Gyr which is smaller than the pulsar characteristic age of 1.8 Gyr. This may indicate that the pulsar age is overestimated. Otherwise, this may be explained by the presence of a thick hydrogen envelope or a low metallicity of the white dwarf progenitor., Comment: 5 pages, 4 figures, accepted for publication in PASA

We report preliminary results of the analysis of the proper motion of the bright radio pulsar B1727$-$47. Using archival Parkes timing data, as well as original and archival ATCA interferometry observations, we, for the first time, constrain the pulsar proper motion at the level of 148$\pm$11 mas yr$^{-1}$. The backward extrapolation of the proper motion vector to the pulsar birth epoch points at the center of the Galactic supernova remnant RCW 114 suggesting the genuine association between the two objects. We discuss the implications of the association and argue that the distance to the system is less than $1$ kpc. This value is at least two times lower than the dispersion measure distance estimates. This suggests that the existing Galaxy electron density models are incomplete in the direction to the pulsar., Comment: 5 pages; 2 figures; Proceedings of the Physics of Neutron Stars - 2017. 50 years after, Saint Petersburg, July 10-14, 2017. In ver. 2, typos corrected

We report preliminary results of the analysis of the proper motion of the bright radio pulsar B1727$-$47. Using archival Parkes timing data, as well as original and archival ATCA interferometry observations, we, for the first time, constrain the pulsar proper motion at the level of 148$\pm$11 mas yr$^{-1}$. The backward extrapolation of the proper motion vector to the pulsar birth epoch points at the center of the Galactic supernova remnant RCW 114 suggesting the genuine association between the two objects. We discuss the implications of the association and argue that the distance to the system is less than $1$ kpc. This value is at least two times lower than the dispersion measure distance estimates. This suggests that the existing Galaxy electron density models are incomplete in the direction to the pulsar., Comment: 5 pages; 2 figures; Proceedings of the Physics of Neutron Stars - 2017. 50 years after, Saint Petersburg, July 10-14, 2017. In ver. 2, typos corrected

We observe a deuteron beam polarization lifetime near 1000 s in the horizontal plane of a magnetic storage ring (COSY). This long spin coherence time is maintained through a combination of beam bunching, electron cooling, sextupole field corrections, and the suppression of collective effects through beam current limits. This record lifetime is required for a storage ring search for an intrinsic electric dipole moment on the deuteron at a statistical sensitivity level approaching 10(-29) e cm., QC 20161005

We present the analysis of the archival Suzaku and Swift X-ray observations of the young $\gamma$-ray pulsar J1932+1916 field. The data revealed a point-like object at the $\gamma$-ray position of the pulsar and diffuse X-ray emission around it. Spectra of the point-like source and diffuse emission are well-described by absorbed power-law models with spectral parameters typical for pulsar plus pulsar wind nebula systems. Therefore we suggest that Suzaku and Swift detected the X-ray counterpart of PSR J1932+1916. Assuming this interpretation, we constrain the distance to the pulsar in the range of 2-6 kpc. We also suggest possible association of the pulsar with the nearby supernova remnant G54.4-0.3 and discuss its implications for the pulsar proper motion, age and distance., Comment: 8 pages, 6 figures, published in MNRAS

We report the results of XMM-Newton observations of the Galactic mixed-morphology supernova remnant G350.0$-$2.0. Diffuse thermal X-ray emission fills the north-western part of the remnant surrounded by radio shell-like structures. We did not detect any X-ray counterpart of the latter structures, but found several bright blobs within the diffuse emission. The X-ray spectrum of the most part of the remnant can be described by a collisionally-ionized plasma model VAPEC with solar abundances and a temperature of $\approx 0.8$ keV. The solar abundances of plasma indicate that the X-ray emission comes from the shocked interstellar material. The overabundance of Fe was found in some of the bright blobs. We also analysed the brightest point-like X-ray source 1RXS J172653.4$-$382157 projected on the extended emission. Its spectrum is well described by the two-temperature optically thin thermal plasma model MEKAL typical for cataclysmic variable stars. The cataclysmic variable source nature is supported by the presence of a faint ($g\approx21$) optical source with non-stellar spectral energy distribution at the X-ray position of 1RXS J172653.4$-$382157. It was detected with the XMM-Newton optical/UV monitor in the $U$ filter and was also found in the archival H$\alpha$ and optical/near-infrared broadband sky survey images. On the other hand, the X-ray spectrum is also described by the power law plus thermal component model typical for a rotation powered pulsar. Therefore, the pulsar interpretation of the source cannot be excluded. For this source, we derived the upper limit for the pulsed fraction of 27 per cent., Comment: 10 pages, 6 figures

We present the analysis of the archival Suzaku and Swift X-ray observations of the young $\gamma$-ray pulsar J1932+1916 field. The data revealed a point-like object at the $\gamma$-ray position of the pulsar and diffuse X-ray emission around it. Spectra of the point-like source and diffuse emission are well-described by absorbed power-law models with spectral parameters typical for pulsar plus pulsar wind nebula systems. Therefore we suggest that Suzaku and Swift detected the X-ray counterpart of PSR J1932+1916. Assuming this interpretation, we constrain the distance to the pulsar in the range of 2-6 kpc. We also suggest possible association of the pulsar with the nearby supernova remnant G54.4-0.3 and discuss its implications for the pulsar proper motion, age and distance., Comment: 8 pages, 6 figures, published in MNRAS

We report the results of XMM-Newton observations of the Galactic mixed-morphology supernova remnant G350.0$-$2.0. Diffuse thermal X-ray emission fills the north-western part of the remnant surrounded by radio shell-like structures. We did not detect any X-ray counterpart of the latter structures, but found several bright blobs within the diffuse emission. The X-ray spectrum of the most part of the remnant can be described by a collisionally-ionized plasma model VAPEC with solar abundances and a temperature of $\approx 0.8$ keV. The solar abundances of plasma indicate that the X-ray emission comes from the shocked interstellar material. The overabundance of Fe was found in some of the bright blobs. We also analysed the brightest point-like X-ray source 1RXS J172653.4$-$382157 projected on the extended emission. Its spectrum is well described by the two-temperature optically thin thermal plasma model MEKAL typical for cataclysmic variable stars. The cataclysmic variable source nature is supported by the presence of a faint ($g\approx21$) optical source with non-stellar spectral energy distribution at the X-ray position of 1RXS J172653.4$-$382157. It was detected with the XMM-Newton optical/UV monitor in the $U$ filter and was also found in the archival H$\alpha$ and optical/near-infrared broadband sky survey images. On the other hand, the X-ray spectrum is also described by the power law plus thermal component model typical for a rotation powered pulsar. Therefore, the pulsar interpretation of the source cannot be excluded. For this source, we derived the upper limit for the pulsed fraction of 27 per cent., Comment: 10 pages, 6 figures

A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune-defined as the number of spin precessions per turn-is given by nu(s) = gamma G (gamma is the Lorentz factor, G the gyromagnetic anomaly). At 970 MeV/c, the deuteron spins coherently process at a frequency of approximate to 120 kHz in the Cooler Synchrotron COSY. The spin tune is deduced from the up-down asymmetry of deuteron-carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the order 10(-8), and to 1 x 10(-10) for a continuous 100 s accelerator cycle. This renders the presented method a new precision tool for accelerator physics; controlling the spin motion of particles to high precision is mandatory, in particular, for the measurement of electric dipole moments of charged particles in a storage ring., QC 20140924

PSR J1357$-$6429 is a young radio pulsar that was detected in X-rays and $\gamma$-rays. We present the high spatial resolution near-infrared imaging of the pulsar field in $J$, $H$ and $K_s$ bands obtained with the VLT/NaCo using the Adaptive Optic system. We found a faint source at the most precise pulsar radio position which we propose as the pulsar near-infrared counterpart candidate. It is confidently detected in the $J$ and $K_s$ bands, with $J$ = 23.51$\pm$0.24 and $K_s$ = 21.82$\pm$0.25. There is a hint of the source in the $H$ band with an upper limit $H$ $>$ 22.8. The dereddened source fluxes are compatible with the extrapolation of the pulsar X-ray spectrum towards the near-infrared. If the candidate is the true counterpart, by this property PSR J1357$-$6429 would be similar to the nearby middle-age pulsar PSR B0656+14. In this case, both pulsars demonstrate an unusually high near-infrared efficiency relative to the X-ray efficiency as compared to other pulsars detected in both ranges., Comment: 6 pages, 4 figures, 3 tables

We present spectral and timing analyses of the X-ray emission from the pulsar wind nebula DA 495 and its central object, J1952.2+2925, suggested to be the pulsar, using archival Chandra and XMM-Newton data. J1952.2+2925 has a pure thermal spectrum which is equally well fitted either by the blackbody model with a temperature of $\approx 215$ eV and an emitting area radius of $\approx 0.6$ km or by magnetized neutron star atmosphere models with temperatures of 80-90 eV. In the latter case the thermal emission can come from the entire neutron star surface which temperature is consistent with standard neutron star cooling scenarios. We place also an upper limit on the J1952.2+2925 nonthermal flux. The derived spectral parameters are generally compatible with published ones based only on the Chandra data, but they are much more accurate due to the inclusion of XMM-Newton data. No pulsations were found and we placed an upper limit for the J1952.2+2925 pulsed emission fraction of 40 per cent. Utilizing the interstellar absorption-distance relation, we estimated the distance to DA 495, which can be as large as 5 kpc if J1952.2+2925 emission is described by the atmosphere models. We compiled possible multi-wavelength spectra of the nebula including radio data; they depend on the spectral model of the central object. Comparing the results with other pulsar plus wind nebula systems we set reasonable constraints on the J1952.2+2925 spin-down luminosity and age. We suggest that the Fermi source 3FGL J1951.6+2926 is the likely $\gamma$-ray counterpart of J1952.2+2925., Comment: 10 pages, 8 figures, accepted for publication in MNRAS

PSR J1357$-$6429 is a young and energetic radio pulsar detected in X-rays and $\gamma$-rays. It powers a compact pulsar wind nebula with a jet visible in X-rays and a large scale plerion detected in X-ray and TeV ranges. Previous multiwavelength studies suggested that the pulsar has a significant proper motion of about 180 mas yr$^{-1}$ implying an extremely high transverse velocity of about 2000 km s$^{-1}$. In order to verify that, we performed radio-interferometric observations of PSR J1357$-$6429 with the the Australia Telescope Compact Array (ATCA) in the 2.1 GHz band. We detected the pulsar with a mean flux density of $212\pm5$ $\mu$Jy and obtained the most accurate pulsar position, RA = 13:57:02.525(14) and Dec = $-$64:29:29.89(15). Using the new and archival ATCA data, we did not find any proper motion and estimated its 90 per cent upper limit $\mu < 106$ mas yr$^{-1}$. The pulsar shows a highly polarised single pulse, as it was earlier observed at 1.4 GHz. Spectral analysis revealed a shallow spectral index $\alpha_{\nu}$ = $0.5 \pm 0.1$. Based on our new radio position of the pulsar, we disclaim its optical counterpart candidate reported before., Comment: 9 pages, 9 figures

PSR J1357$-$6429 is a young radio pulsar that was detected in X-rays and $\gamma$-rays. We present the high spatial resolution near-infrared imaging of the pulsar field in $J$, $H$ and $K_s$ bands obtained with the VLT/NaCo using the Adaptive Optic system. We found a faint source at the most precise pulsar radio position which we propose as the pulsar near-infrared counterpart candidate. It is confidently detected in the $J$ and $K_s$ bands, with $J$ = 23.51$\pm$0.24 and $K_s$ = 21.82$\pm$0.25. There is a hint of the source in the $H$ band with an upper limit $H$ $>$ 22.8. The dereddened source fluxes are compatible with the extrapolation of the pulsar X-ray spectrum towards the near-infrared. If the candidate is the true counterpart, by this property PSR J1357$-$6429 would be similar to the nearby middle-age pulsar PSR B0656+14. In this case, both pulsars demonstrate an unusually high near-infrared efficiency relative to the X-ray efficiency as compared to other pulsars detected in both ranges., Comment: 6 pages, 4 figures, 3 tables

We present spectral and timing analyses of the X-ray emission from the pulsar wind nebula DA 495 and its central object, J1952.2+2925, suggested to be the pulsar, using archival Chandra and XMM-Newton data. J1952.2+2925 has a pure thermal spectrum which is equally well fitted either by the blackbody model with a temperature of $\approx 215$ eV and an emitting area radius of $\approx 0.6$ km or by magnetized neutron star atmosphere models with temperatures of 80-90 eV. In the latter case the thermal emission can come from the entire neutron star surface which temperature is consistent with standard neutron star cooling scenarios. We place also an upper limit on the J1952.2+2925 nonthermal flux. The derived spectral parameters are generally compatible with published ones based only on the Chandra data, but they are much more accurate due to the inclusion of XMM-Newton data. No pulsations were found and we placed an upper limit for the J1952.2+2925 pulsed emission fraction of 40 per cent. Utilizing the interstellar absorption-distance relation, we estimated the distance to DA 495, which can be as large as 5 kpc if J1952.2+2925 emission is described by the atmosphere models. We compiled possible multi-wavelength spectra of the nebula including radio data; they depend on the spectral model of the central object. Comparing the results with other pulsar plus wind nebula systems we set reasonable constraints on the J1952.2+2925 spin-down luminosity and age. We suggest that the Fermi source 3FGL J1951.6+2926 is the likely $\gamma$-ray counterpart of J1952.2+2925., Comment: 10 pages, 8 figures, accepted for publication in MNRAS

PSR J1357$-$6429 is a young and energetic radio pulsar detected in X-rays and $\gamma$-rays. It powers a compact pulsar wind nebula with a jet visible in X-rays and a large scale plerion detected in X-ray and TeV ranges. Previous multiwavelength studies suggested that the pulsar has a significant proper motion of about 180 mas yr$^{-1}$ implying an extremely high transverse velocity of about 2000 km s$^{-1}$. In order to verify that, we performed radio-interferometric observations of PSR J1357$-$6429 with the the Australia Telescope Compact Array (ATCA) in the 2.1 GHz band. We detected the pulsar with a mean flux density of $212\pm5$ $\mu$Jy and obtained the most accurate pulsar position, RA = 13:57:02.525(14) and Dec = $-$64:29:29.89(15). Using the new and archival ATCA data, we did not find any proper motion and estimated its 90 per cent upper limit $\mu < 106$ mas yr$^{-1}$. The pulsar shows a highly polarised single pulse, as it was earlier observed at 1.4 GHz. Spectral analysis revealed a shallow spectral index $\alpha_{\nu}$ = $0.5 \pm 0.1$. Based on our new radio position of the pulsar, we disclaim its optical counterpart candidate reported before., Comment: 9 pages, 9 figures

We present results of the spectral analysis of the X-ray emission from the middle-aged Fermi pulsar J1741$-$2054 using all Chandra archival data collected in 2010 and 2013. We confirm early findings by Romani et al. 2010 that the pulsar spectrum contains a thermal emission component. The component is best described by the blackbody model with the temperature $\approx$ 60 eV and the emitting area radius $\approx$ 17 $D_{\rm kpc}$ km. The thermal emission likely originates from the entire surface of the cooling neutron star if the distance to the pulsar is $\approx$ 0.8 kpc. The latter is supported by a large absorbing column density inferred from the X-ray fit and empirical optical extinction--distance relations along the pulsar line of sight. The neutron star surface temperature and characteristic age make it similar to the well studied middle-aged pulsar B1055$-$52. Like the latter PSR J1741$-$2054 is hotter than the standard cooling scenario predicts., Comment: 8 pages, 5 figures, Accepted to ApJ

We present results of the spectral analysis of the X-ray emission from the middle-aged Fermi pulsar J1741$-$2054 using all Chandra archival data collected in 2010 and 2013. We confirm early findings by Romani et al. 2010 that the pulsar spectrum contains a thermal emission component. The component is best described by the blackbody model with the temperature $\approx$ 60 eV and the emitting area radius $\approx$ 17 $D_{\rm kpc}$ km. The thermal emission likely originates from the entire surface of the cooling neutron star if the distance to the pulsar is $\approx$ 0.8 kpc. The latter is supported by a large absorbing column density inferred from the X-ray fit and empirical optical extinction--distance relations along the pulsar line of sight. The neutron star surface temperature and characteristic age make it similar to the well studied middle-aged pulsar B1055$-$52. Like the latter PSR J1741$-$2054 is hotter than the standard cooling scenario predicts., Comment: 8 pages, 5 figures, Accepted to ApJ

Context. PSR J1048-5832 is a young radio-pulsar that has recently been detected in gamma-rays with Fermi, and also in X-rays with Chandra and XMM-Newton. It powers a compact pulsar wind nebula visible in X-rays and is in many ways similar to the Vela pulsar. Aims. We present deep optical observations made with the ESO Very Large Telescope to search for optical counterparts of the pulsar and its nebula and to explore their multi-wavelength emission properties. Methods. The data were obtained in the V and R bands and were compared with archival data in other spectral domains. Results. We do not detect the pulsar in the optical and derive informative upper limits of R greater than or similar to 28(.)(m) 1 and V less than or similar to 28(.)(m) 4 for its brightness. Using a red-clump star method, we estimate an interstellar extinction towards the pulsar of A(V) approximate to 2 mag, which is consistent with the absorbing column density derived from X-rays. The respective distance agrees with the dispersion measure distance. We reanalysed the Chandra X-ray data and compared the dereddened upper limits with the unabsorbed X-ray spectrum of the pulsar. We find that regarding its optical-X-ray spectral properties this gamma-ray pulsar is not distinct from other pulsars detected in both ranges. However, like the Vela pulsar, it is very inefficient in the optical and X-rays. Among a dozen optical sources overlapping with the pulsar X-ray nebula we find one with V approximate to 26(.)(m)9 and R approximate to 26. m 3, whose colour is slightly bluer than that of the field stars and is consistent with the peculiar colours typical for pulsar nebula features. It positionally coincides with a relatively bright feature of the pulsar X-ray nebula, resembling the Crab wisp and is located in similar to 2 '' from the pulsar. We suggest this source as a counterpart candidate to the feature. Conclusions. Based on the substantial interstellar extinction towards the pulsar and its optical i, AuthorCount:5

(Abridged) PSR J1357-6429 is a Vela-like radio pulsar that has been recently detected in X-rays and gamma-rays. It powers a compact tail-like X-ray pulsar wind nebula and X-ray-radio plerion associated with an extended TeV source HESS J1356-645. We present our deep optical observations with the Very Large Telescope to search for an optical counterpart of the pulsar and its nebula. We detect a point-like source in V, R, and I bands whose position is within the 1-sigma error circle of the X-ray position of the pulsar, and whose colours are distinct from those of ordinary stars. We consider it as a candidate optical counterpart of the pulsar. If it is indeed the counterpart, its 5-sigma offset from the radio pulsar position, measured about 9 yr earlier, implies that the transverse velocity of the pulsar is in the range of 1600--2000 km s^{-1} at the distance of 2--2.5 kpc, making it the fastest moving pulsar known. The direction of the estimated proper motion coincides with the extension of the pulsar's X-ray tail, suggesting that this is a jet. The tentative optical luminosity and efficiency of the pulsar are similar to those of the Vela pulsar, which also supports the optical identification. However, the candidate undergoes an unusually steep dereddened flux increase towards the infrared with a spectral index of about 5, that is not typical of optical pulsars. It implies a strong double-knee spectral break in the pulsar emission between the optical and X-rays. The reasons for the spectral steepness are unclear. It may be caused by a nebula knot projected onto the jet and strongly overlapping with the pulsar, as observed for the Crab, where the knot has a significantly steeper spectrum than the pulsar. We find no other signs of the pulsar nebula in the optical. Alternatively, the detected source may be a faint AGN, that has not yet been seen at other wavelengths., Comment: 13 pages, 10 figures, 4 tables